Release process holds promise for GaN semiconductors

TOKYO – Nitride semiconductors grow only on certain surfaces, and their utility is limited by the substrate on which they are fabricated. But a new release process not only makes the method cheaper and easier, it also expands the potential uses of the materials.

Yasuyuki Kobayashi and colleagues at Nippon Telegraph and Telephone Corp. (NTT) demonstrated the process with a technique called mechanical transfer using a release layer (MeTRe). They grew a very thin hexagonal layer of boron nitride (h-BN) between a sapphire substrate and a gallium nitride (GaN)-based semiconductor. Sandwiched in the middle, the h-BN works as a release layer, allowing the investigators to easily detach the semiconductor and transfer it to other substrates without using expensive laser beam machining or chemical treatment.

Comparison of the traditional methods of fabricating GaN semiconductors and the technique devised by Yasuyuki Kobayashi and colleagues. Courtesy of NTT Science and Core Technology Laboratory Group.
GaN-based semiconductors have a wide range of applications in high-power electronic devices and light sources, but their utility is hampered by the thickness of the substrates on which they are grown. The substrates are hard to separate from the GaN and can be 100 times the size of the film. They also must be stable at high temperatures because the GaN-based film’s growth temperature is 1000 °C. Using the MeTRe fabrication process, GaN-based thin-film devices can be separated easily from their substrate and transferred onto other devices, and they can be grown on nearly any single crystal substrate. Both of these qualities greatly increase their utility.

Boron nitride also is difficult to grow on a single-crystal sapphire substrate because of a very different crystal structure. However, the researchers optimized its growth using metallorganic chemical vapor deposition, which uses the constituent gases to encourage single-crystal, thin-film growth on the substrate’s surface. They also found that the GaN layers could be grown on top of the BN film if a buffer layer of Al1—xGaxN, an aluminum/gallium nitride alloy, was used.

The MeTRe method of semiconductor fabrication is cheaper, faster and easier than conventional methods; a worldwide movement has been under way to develop such an efficient technique. The process also allows thin, flexible semiconductors with a large surface area (up to 2 cm) to be made. One key application of such a semiconductor is in the development of flexible solar panels that are sensitive to UV wavelengths only and that can be put in windows to filter out the harmful rays while also collecting and storing solar energy. This can be accomplished easily by attaching GaN-based solar cells to pre-existing silicon-based ones.